1. Field of the Invention
The present invention relates to an optical device for use in opto-electrical integrated circuits and the like which are needed in fields of optical communication, and particularly, to an optical device with an optical coupler formed in a channel waveguide for effecting light wave branching and/or combining by splitting a wavefront of a propagating light wave. The present invention also relates to a method for forming an optical coupler in a channel waveguide of an optical device.
2. Related Background Art
In recent years, there has been presented an interferometric laser, which is a kind of compound-resonator laser including a Y-branch optical coupler 200 as shown in FIG. 1. The interferometric laser comprises a structure of layers layered as shown in FIG. 1 (see I. H. A. Fattah et al. "Semiconductor Interferometric Laser" Appl. Phys. Lett. 41, 2, pp. 112-114 (July 1982)).
Further, interferometric lasers respectively including X-branch optical couplers 210a and 210b as shown in FIGS. 2A and 2B have also been presented (see J. Salzman et al. "Cross Coupled Cavity Semiconductor Laser" Appl. Phys. Lett. 52, 10, pp. 767-769 (March 1988)). In FIGS. 2A and 2B, reference letters R.sub.1 -R.sub.4 designate resonance or cavity surfaces, reference letters L.sub.1 -L.sub.4 designate resonator lengths and reference letters r and t respectively indicate light reflection and transmission.
These two prior art devices, however, have the following disadvantages.
In the device of FIG. 1 including the Y-branch optical coupler 200, there are problems that the branching angle of the Y-branch coupler 200 cannot be made large and that the size of such device will inevitably be large (i.e., the length of the device will be more than 1 mm) compared with other optical devices, so that integration thereof with other devices is difficult to realize.
In the devices of FIGS. 2A and 2B including the X-branch optical couplers, there are problems that high process accuracies such as a high position accuracy and a high depth accuracy are required in producing the X-branch couplers 210a and 210b for achieving desired reflection and transmission efficiencies, so that yield as well as reproducibility of such devices is low. That is, the property of light branching and combining is determined by how the X-branch coupler is formed relatively to a field distribution or field amplitude variation of a light wave propagating in a light waveguide. Therefore, the process accuracies need to be high and strict considering the state of the field distribution of light.
A more detailed explanation will now be made of the prior art devices of FIGS. 2A and 2B, referring to FIGS. 3, 4A and 4B. The optical coupler for effecting branching and combining of a light wave propagated in crossing waveguides is produced by forming a fine slit 211 extending in a direction of layered layers or a vertical direction, at a cross portion of waveguides 212 and 212' as shown in FIG. 3.
However, in the prior art device, there is a light confinement structure in a direction parallel to a plane defined by the layered layers or a lateral direction of the light waveguide (typically, a ridge type structure), so that the cross portion before the slit 211 is formed is not flat. As a result, the tip depth of the slit 211 at its central portion B is greatly different from that at its opposite end portions A and C, as shown in FIG. 4B. The difference in the tip depth of the slit 211 makes it difficult to control the coupling efficiency and the branching ratio of light because the field distribution 219 of the propagating light wave generally expands wider than the width of the light waveguide, as shown in FIG. 4B.
In FIGS. 4A and 4B respectively showing cross-sections of FIG. 3 taken along a line A--A' and a line B--B', reference numeral 213 is a substrate, reference numerals 214 and 216 are clad layers, reference numeral 215 is a core layer of the waveguide, reference numerals 217 and 219 are respectively field distributions in the light waveguide with respect to A--A' and B--B' cross sections and reference numeral 218 is a ridge portion for forming the light waveguide. When the slit 211 is formed at the cross portion of the ridge waveguide by focused ion beam etching (FIBE), reactive ion beam etching (RIBE) or the like, the tip depth of the slit 211 varies in the lateral direction. As a result, even if the tip depth of the slit 211 is set to a central portion of the field distribution at its central part B so that the slit 211 divides the field distribution 219 at its central portion in the direction of layered layers, the tip depth becomes too deep at opposite end parts A and C since no ridge portion 218 is there. Hence, the reflection efficiency becomes large differently from a desired one. Thus, it is extremely difficult to control the tip depth of the slit 211 for obtaining a desired branching efficiency. That is, it is difficult to control a splitting ratio or ratio of transmission/reflection by the tip position of the slit 211.
Further, an irregularity of the tip depth of a slit more or less exists in an embedded waveguide structure since a step or the like is created at the time of an embedding process.